Near Europa's south pole are chains of scalloped lines that run for hundreds of kilometres across the frozen, fractured surface.

Until now, there have been no good ideas as to what formed these "cycloidal" features, or "flexi," as they have been officially named.

Each crack takes just 3.5 days to form

But planetary scientists at the University of Arizona in Tucson have just put forward a very interesting theory, which also provides what is perhaps the most convincing argument yet for the existence of an under-ice ocean on the moon.

The suggestion is that Europa has a 160-kilometre-thick layer of water - 20 times thicker than the Earth's oceans - but the top layer is frozen. Many scientists believe that this ocean would be a good place to look for life.

Daily tides

The astronomers suggest that the cycloidal cracks form in Europa's solid-ice surface with the daily rise and fall of tides in the subsurface waters.

Cracks in the ice and possibly an ocean beneath

The tidal stresses induced by mighty Jupiter cause Europa's ocean to ebb and flow as it orbits Jupiter. According to the researchers, Europa's ocean tides rise and fall a distance of 30 meters.

When tidal stress reaches the tensile strength of ice, a crack will start to appear. The crack propagates relatively slowly across the surface following a curving path until the stress drops below the tensile strength of the ice, when it stops.

A few hours later, when tidal stress again exceeds the tensile strength of ice, the crack begins a new curve in another direction.

"You could probably walk along with the advancing tip of a crack as it was forming - if you could survive Europa's radiation environment," researcher Dr Gregory Hoppa said. "And while there's not enough air to carry sound, you would definitely feel vibrations as it formed."

Mystery features

One of their most striking conclusions is that each arc segment forms in 3.5 days - the time it takes Europa to make one complete orbit around Jupiter.

"What amazes me about this is just how long these features have been a mystery," Hoppa adds.

"We've been staring at pictures of them for 20 years, since the Voyager spaceprobe sent back pictures in 1979. We did not know what made them. And it seems what they've been telling us all along is that an ocean was there when these things formed."